A vehicle power unit is known that includes a twin clutch type transmission having a pair of main shafts disposed in parallel with each other, countershafts having axes that extend in parallel with the main shafts, gear trains of a plurality of shift speeds disposed between the main shafts and the countershafts, each of the gear trains being selectively established, and a clutch, disposed coaxially on each of the main shafts, for connecting and disconnecting power transmission from a crankshaft. See, for example, Japanese Patent Laid-open No. 2008-303939. This arrangement is more advantageous than in an arrangement in which a pair of clutches is disposed in tandem coaxially, in that an axial width of the power unit can be prevented from increasing.
The above-described known arrangement having a pair of main shafts and clutches mounted on mutually different shafts tends to require a greater space for disposing the shafts, as compared with the arrangement having main shafts and clutches on an identical axis. This poses a problem in that a vehicle body size is affected, if the power unit is applied to a compact vehicle (saddle riding type vehicle), such as a motorcycle.